Microfluidic pumping and dispensing of liquid chemical reagents is the subject of three U.S. Pat. Nos. 5,585,069; 5,593,838; and 5,603,351; all assigned to the David Sarnoff Research Center, Inc. and hereby incorporated by reference. The system uses an array of micron sized reservoirs, with connecting microchannels and reaction cells etched into a substrate. Electrokinetic pumps include electrically activated electrodes within the capillary microchannels provide the propulsive forces to move the liquid reagents within the system. The electrokinetic pump, which is also known as an electroosmotic pump, has been disclosed by Dasgupta et al, see "Electroosmosis: A Reliable Fluid Propulsion System for Flow Injection Analyses", Anal. Chem. 66, pp 1792-1798 (1994). The chemical reagent solutions are pumped from a reservoir, mixed in controlled amounts, and then pumped into a bottom array of reaction cells. The array could be decoupled from the assembly and removed for incubation or analysis. When used as a printing device, the chemical reagent solutions are replaced by dispersions of cyan, magenta, and yellow pigment, and the array of reaction cells could be considered a viewable display of picture elements, or pixels, comprising mixtures of pigments having the hue of the pixel in the original scene. When contacted with paper, the capillary force of the paper fibers pulls the dye from the cells and holds it in the paper, thus producing a paper print, or reproduction, of the original scene.
One problem known to printing is an image artifact called printing non-uniformities. Printing non-uniformities can be produced by different causes. For example, many printing apparatus transport a receiver relative to the print head during printing. Non-uniform mechanical movement in the motors or gears often produces "banding" type of printing non-uniformities. These mechanical transport related printing non-uniformities are overcome by above referenced, commonly assigned U.S. Patent Applications that disclosed microfluidic printing apparatus comprising two-dimensional array of microfluidic mixing chambers. An image area is formed on a receiver when the receiver is in contact with the printing apparatus. The ink delivery chambers are not required to move relative to the receiver during the ink transfer.
Another cause for printing non-uniformities is the variabilities between the ink delivery means of different pixels. For a microfluidic printing apparatus, the variabilities between the ink delivery means such as ink mixing chambers can be the variabilities in the volumes of the ink mixing chambers, the diameter of the ink supply channels, or the pumping efficiencies of the electrokinetic pumps. These variabilities are often introduced in the micro-fabrication process of the microfluidic printing apparatus. Variabilities between ink mixing chambers result in pixel-wise variabilities in the amounts of ink delivered even if a uniform input image is printed. The variability problem is particularly severe for microfluidic printing apparatus comprising a large number of mixing chambers in a two-dimensional array because it is usually more difficult to control variabilities in the micro-fabrication process involving a large number of mixing chambers.